Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

1991 Mazda Miata MX-5 running rich, no codes

Runs very rich. No check engine light or trouble codes. O2 sensor and fuel pressure regulator have been changed with no results.

One thing to check is the coolant temp sensor for the computer.

If the sensor is faulty where it’s telling the computer that the coolant temperature never gets above zero degrees, the computer will command that the engine runs rich.


Is the wiring from this sensor (three wires to connector) located next to the connector for the O2 sensor? I know this sensor is impossible to see unless the coil pack is removed (real pain). I have heard that the voltage is supposed to be 2.2 when cold and .4 at normal operating temperature. Does this sound correct? Just wondering if I test the voltage in that connector, I’ll be testing the correct thing.

Have not had this car very long. Had it diagnosed at the dealer and temp sensor is OK plus plugs are nice and brown but the cat is original and has taken an impact (minor) so is wasted. On my to do list. There is a new cat back exhaust so I suspect someone may have hollowed out the cat while doing the exhaust work.